The invention at hand relates to a micro-filter disc, particularly for the sterilization of water, and a method for its production.
During the mechanical sterilization of water by means of a microfilter, it is essential that the pore size of the filter device be small enough and be of a typical size of 0.4 to 0.5.mu. so that bacteria are safely re-trained. Ceramics or other materials having suitable pore sizes, up to now, could not be produced in just any desirable form so that filters with large surfaces and having good filtering effectiveness could only be produced under large expenditures and poor serviceability. The until now used large surface area filter columns and having been made of ceramics are difficult to clean, whereby known filter installations, commonly, have to be opened frequently and have to be cleaned by hand.
Reference is particularly made to the European patent application 92 810 843, wherein a self-cleaning filter installation is being described having been provided with a cleaning unit being moveable relative to the filter surfaces and being movable thereagainst for the scraping off of filter material. Such filters can be regularly cleaned in an assembled state by cleaning brushes being driven from the outside to thereby substantially simplify the maintenance.
An object of the invention at hand is the provision of a flat micro-filter disc having a large surface area which, for example, can be installed in a self-cleaning filtering installation. Furthermore, a suitable method for the production of such a micro-filter disc is indicated.
To attain this object, a micro-filter disc is provided which is produced on a kieselguhr (diatomaceous earth) basis and contains at least two silicately bonded kinds of kieselguhr of which at least one is pre-calcined and later being sintered, and whereby the micro-filter disc exhibits of pore size in the magnitude of 0.05 to 0.5.mu., preferably 0.1 to 0.3.mu.. It has been discovered that the presence of a kind of a pre-calcined kieselguhr assures the stability of the micro-filter disc during its production and forms a framework for the provision of at least a further kind of kieselguhr of a different kind, whereby a stable form is obtained having the desired pore sizes. Particularly, a monolithical round microfilter disc can be produced having a diameter of at least 25 cm, preferably at least 35 cm. Such a disc can have a thickness of 0.5 to 2 cm, preferably about 1 to 1.5 cm. A good filtering effect is obtained with a simultaneous good breaking strength of the disc having a thickness of about 0.5 cm. A typical permeability of the disc, according to the invention, stands at 0.5 to 0.7 liter (midm.sup.2), preferably at 0.6.
It is preferred that the micro-filter disc has a central round opening having a diameter of 3 to 15 cm, preferably 6 to 10 cm. Because of the presence of this opening, several micro-filter discs can be superimposed upon each other on a central column, wherein the column can rotate or have a rotatable shaft to facilitate a relative movement between the micro-filter discs.
In order to be able to tightly install such micro-filter discs in a filtering installation, the outer and inner margins of the micro-filter disc have to be cut to exact tolerances which can be obtained by high pressure water-jet cutters. The margins are preferably provided with a sealing layer which also overlaps around the disc on at least one side by about 1 to 8 mm, preferably by 3 to 5 mm. In this manner, the disc can be mounted sealingly with its margins as well as with an underlayer. The seals should have a thickness of 1 to 3 mm in order to accommodate any size changes of the disc.
It is preferred that the filter disc exhibit open pores on its upper surface to enhance an automatic cleaning of the upper surface from time to time, which surface should be soft enough to obtain an abrading by means of a cleaning brush. Such cleaning brush can have man-made bristles, for example, particularly nylon or precious metal bristles, whereby the aim is to obtain an upper surface cleaning by abrading a layer having a thickness of about 0.1 to 5.mu. with just a few working strokes. This abrading should be obtained after about one to twenty relative movements between the upper surface and the cleaning brush. The abraded substance can thereafter be rinsed away during a rinsing cycle and will not permeate through the filter disc.
In order to kill invading bacteria and in order to avoid the growth of bacteria retained on the upper surface through the micro-filter disc, the disc preferably contains parts of a non-washable silver oxide. A disc having especially great stability and good filtering effectiveness can be obtained through the use of least three different kinds of kieselguhr and a portion of kaolin. One kind of the kieselguhr is pre-calcined and at least one other kind contains particularly many small particles, about 15% of the particles having a size below 2.mu..
In a preferred embodiment, the micro-filter disc contains 6 to 12 percent by weight, preferably 9 percent by weight of a pre-calcined and commonly with the other components again sintered kieselguhr, 25 to 30 percent kaolin, and the remainder being a further kinds of kieselguhr as well as possibly silver oxide and other smaller amounts of production mandated components. The percent by weight indicator relates to the weight of the finished sintered and dried disc.
The production of such a micro-filter disc is undertaken by way of a method involving the following steps:
a. The following is being mixed: At least 6 to 12 parts by weights, preferably 9 parts by weight, of an already calcined kind of kieselguhr, 25 to 35 parts by weight, preferably 30 parts by weight of kaolin and 53 to 69 parts by weight, preferably 61 parts by weight, of a further non- or at least very little calcined kind of kieselguhr; PA1 b. The mixture is stirred-up with 200 to 250 parts by weight, preferably 225 parts by weight of water to which is added 0.2 to 0.35 parts by weight, preferably 0.27, a liquefying agent to decrease the surface tension and an organic binding agent at 0.4 to 0.8 parts by weight, preferably 0.5 to 0.7 parts by weight; PA1 c. The mixture is being stirred until a definite change in viscosity occurs, preferably in 12 to 15 hours; PA1 d. The stirred and finished mixture is poured into a mold having been made of a moisture absorbing material such as plaster of paris; PA1 e. The mixture is slowly being dried, preferably over a period fo time of 8 to 25 days, particularly 10 to 20 days; PA1 f. The dried mixture is now being sintered by being burnt at a temperature of 1000.degree. to 1050.degree. C., particularly at 1035.degree. C.
By way of examples and as will be explained later on, the liquefying agent in combination with the prolonged stirring results in a very even distribution of the fine particles of the differing kinds of kieselguhr and the kaolin throughout the mixture. The organic binding agent aids in the coherence of the unsintered form while drying inside or outside the mold, while the already calcined portion of the kieselguhr forms a stable framework and thereby assures a form stability (green strength) of the still unsintered disc. Stabilized by the portion of the calcined kieselguhr, and held together by the organic binding agent, the remaining components fall in line in a very homogenous order, whereby the desired porosity is obtained. In this manner, it is possible to produce monolithic discs having the desired porosity and diameters not achievable until now and free of cracks.
In a preferred example, the following components are being used expressed in parts by weight:
About 40.72 of a first kind of non-calcined or at least very little calcined kieselguhr, about 9.05 of a second kind of calcined kieselguhr, about 20.82 of a third kind of a non-calcined or at least very title calcined kieselguhr, and about 29.41 of kaoline.
Herein, kinds of kieselguhr are employed having differing distributions of particle sizes, so that as a whole, a desired porosity in the disc in the magnitude of 0.05.mu. to 0.5.mu., preferably 0.1 to 0.3.mu., is obtained.
In order to avoid any bacteria being retained on the upper surface and later on being able to grow through the ceramic structure, silver oxide is added to the mixture at 0.2 to 0.5 parts per weight, preferably at about 0.36.
In order to avoid any cracking of the micro-filter disc during its production, the choice of the mold is very important. It is preferred that the material for the mold should be minimally moist plaster of paris into which mold the finished mixture is being poured. Plaster of paris is a material being water absorbable. When a completely dry plaster of paris mold is used, it absorbs water rather quickly from the mixture and can result in a possible development of cracks. Therefore, a minimally moist plaster of paris is preferred.
In order to produce discs having openings through the center, the form must be provided with a control core which is instrumental in forming the opening. Also, in this area, formation of cracks can easily occur. Therefore, the core should consist of a yieldable and absorbent material which yields to any deforming of the disc while it is drying. A hollow cylinder made of paper or card board is particularly useful because it initially absorbs moisture but during drying of the mixture later on can undergo a minimal deforming and, finally, is easily separable from the disc.
During the process of sintering the thus pre-manufactured and pre-dried disc, there is commonly an increase in the density of the upper surface which for the intended purpose, namely, the filtering of fluids, is not permeable enough and, therefore, unsuitable. Therefore, after sintering, the denser upper layer is being removed by a scraping tool, which layer commonly has a thickness of about 5.mu..
Finally, the disc may be cut to an exact measure. This can be done by the use of a high pressure water-jet cutters. In order to be able to install the discs in a sealing manner in a filter installation, the outer and inner margins of each disc are provided with sealing layers which overlap onto at least one side of the disc. The sealing layer material is preferably based on silicone which can be glued to the disc.
Following is a detailed description of the invention with reference being made to the drawings and to particular examples with the attendant advantages and modifications.